Spinning Nettles fibres is much the same as spinning any other fibre, the method of spinning it depends on the length of the fibre, the quality, and what you want to use the yarn for, as well as the treatment you intend to give the yarn after it has been spun.
The difficulty with the nettle is extracting the fibre from the stems. Traditionally this has been done in the same way as Flax and Hemp, i.e. with water or dew retting. This is a long and strenuous process, but a wonderful challenge, and there are no rules, unless you are an archaeologist trying to imitate historical methods.